The most widely used optical technique for measurement of velocity of fluid flow or solid objects known in the art is the laser Doppler technique. According to this technique, two laser beams from the same laser are set to intersect at their beam waists. This region is referred to as the measurement volume. Scattered light from particles passing this region is imaged onto a photo detector. Since the light scattered from both beams reaches the detector simultaneously, a beat frequency, corresponding to the difference in Doppler shifts from the two scattered beams, is obtained. The beat frequency is directly proportional to the velocity component perpendicular to the fringe geometry, which emerges in the cross section. The velocity is determined by Fourier transformation and/or counting zero crossings of the high-pass filtered detector signal. The principle is often referred to as LDA, when used for measuring velocities of gases, or Laser Doppler Velocimetry (LDV), when used for measuring velocities of fluids or solid objects.
The most common configuration of the LDA system is the differential LDA system—see FIG. 1. A laser beam is split into two beams of equal intensity by a 50/50 equal path length beam splitter and later both beams are focused and crossed at the point under investigation by a condensing lens. Scattered light from particles passing through this region is focused and sent to a photo detector. Since the light scattered from both beams reaches the detector simultaneously, a beat frequency, corresponding to the difference in Doppler shifts from the two scattered beams, is obtained. The beat frequency is directly proportional to the velocity component perpendicular to the fringe geometry, which emerges in the cross section.
LDA has found many applications, especially in the studies of fluid dynamics, where the light does not obstruct the flow. Furthermore, the spatial resolution of LDA systems is high. Nothing other than light has to be sent to the point of interest (measurement volume) and the light beams can be focused to a very small measurement volume.
Traditionally, light sources in LDA systems are gas lasers e.g. a Helium-Neon laser or an Argon-ion laser. The use of such bulky laser systems makes conventional LDA systems rather bulky and expensive in terms of manufacturing.
One of the steps in making LDA systems compact and cheap is to use semiconductor devices as the light source—for instance a laser diode. However, the emission wavelength of laser diodes is significant temperature dependent and even temperature stabilised laser diodes will be undefined within 1-3 longitudinal modes due to mode hops and hysteresis in the temperature dependence.
Consequently, the fringe spacing Λ in the measurement volume will vary due to a linear dependency on the wavelength,   Λ  =      λ          2      ⁢                           ⁢      sin      ⁢                           ⁢              (                  θ          /          2                )            where λ is the wavelength and θ is the angle between the probe beams. Since the measured frequency and thereby also the estimated velocity is directly proportional to the fringe spacing, the measured frequency for a given velocity will vary with the wavelength and will thus yield uncertainties in the estimation of the velocity.
From U.S. Pat. No. 4,948,257 a laser optical measuring device according to the laser Doppler principle is known. A method for stabilizing the fringe pattern in the measurement volume is described. Variations in the fringe spacing due to wavelength changes are compensated for by changing the closing angle of deflected beams.
A disadvantage of the system of U.S. Pat. No. 4,948,257 is that the closing angle of the beams in the measurement volume must be equal to the closing angle of the grating used as the beamsplitter. This results in strong limitations on the obtainable fringe spacing in the measurement volume and the physical dimension of the optical system cannot be less than the working distance (distance from last optical element to the measurement volume). In an embodiment, this problem is partly solved by introducing at least three specially designed prisms. However, the additional prisms increases the complexity and cost of manufacturing of the system of U.S. Pat. No. 4,948,257.
Furthermore, in one of the embodiments the position of the measurement volume will strongly depend on the wavelength which is undesirable. In addition, no optical receiver system for collecting the scattered light has been devised and the optimal back-scatter receiver will be difficult to implement.
From PCT No. WO 00/19212 a miniature laser Doppler probe is known. The fringe pattern in the measurement volume is formed through imaging of a diffraction grating which makes it substantially independent of wavelength. The imaging system consists of a single focusing lens. Only at a single position of this lens an image will be generated, where the beam waists are coincident with the image plane. This is needed in a laser Doppler system in order to obtain parallel fringes in the measurement volume.
Thus, in the system of WO 00/19212 the grating and lens set a strong limitation of the obtainable fringe period in the measurement volume. In order to obtain a better system a clean imaging system, consisting of at least two lenses, placed with a distance equal to the sum of the focal lengths is required. Such modification will add to the complexity and cost of the system suggested in WO 00/19212.
From J. Schmidt et al, “Diffractive beamsplitter for laser Doppler velocimetry”, Opt. Letts. 17, 1240-1242, (1992), a diffractive beamsplitter arrangement for use in LDV is known. Here, the achromatic behaviour of diffractive gratings is used to make the fringe period in the measurement volume independent of the wavelength. In order to obtain high overall efficiency in the system (75%), the diffraction gratings are implemented as multiplexed volume holograms in dichromated gelatine. It is not possible to replicate such multiplexed volume holograms in a large scale low cost production.
The use of surface relief gratings would make replication of the diffraction gratings easy through e.g. injection moulding in polymer materials. However, using such surface relief gratings in the LDV suggested by J. Schmidts et al. would make the overall system efficiency much lower than the 75% obtained using volume holograms. This is due to the fact that is not possible to realise high diffraction efficiencies of surface relief gratings with the small diffraction angles and grating frequencies used here.
It is an object of the present invention to provide a flexible and simple beamsplitter arrangement that solves the above mentioned problems and is suitable for being used in highly efficient and low cost LDA systems.